U.K. Measles Threat Menaces U.S.

Nov. 12, 2003 -- A major measles outbreak in England? With vaccination rates dropping across the U.K., it may be only a matter of time.

Measles vaccine is part of the MMR (measles-mumps-rubella) combination vaccine. It's extremely safe, agrees every major health agency in the world. But now-refuted claims that the vaccine is linked to autism persist. In the U.K., that's led to a drop in MMR coverage to 82% from a high of 92% in 1995-1996.

A 90% coverage rate ensures that outbreaks will quickly burn themselves out. The U.K.'s low MMR vaccination rate means that a single case of measles could explode into a major outbreak, says Steve Cochi, MD, MPH, deputy director of the CDC's National Immunization Program.

"Formerly, a case of measles imported into the U.K. was like throwing a match into a wet forest," Cochi tells WebMD. "Now it's like throwing a match into California."

The Criticality Threshold

A vaccine coverage rate above 80% seems pretty high. But that's just a national average. It means that two in 10 people are vulnerable to this highly infectious disease. And it suggests that there must be many pockets of unprotected people. In some areas of London, MMR coverage has dropped to 64%.

The key is something called the reproductive number, says Vincent A.A. Jansen, PhD, a professor at Royal Holloway University of London.

"The reproductive number is the typical number of cases an infected individual would cause," Jansen tells WebMD. "If it's less than 1, the outbreak would fizzle out. If it is greater than 1, the epidemic can take off. So 1 is the threshold of criticality."

The U.K. is now close to that threshold. It's climbed from 0.47 in 1998 to 0.82 in 2002. At that rate, England will soon cross the threshold of criticality.

Could It Happen Here?

Even at 91%, the U.S. measles vaccination rate isn't as high as experts would like. Especially if there's an epidemic in England, imported cases could cause relatively large outbreaks in some areas. Across the nation, however, more than nine in 10 2-year-olds have had their first dose of measles vaccine, making a large U.S. outbreak extremely unlikely.